The positioning and the orientation of a spacecraft with respect to the earth and to space are essential parameters in the operation of the craft. In the case of a satellite, one seeks on the one hand to control the orbit of the satellite around the earth through the six orbital parameters (semi-major axis, eccentricity, inclination, argument of the ascending node, argument of the perigee and true anomaly). One also seeks to know or indeed stabilize the orientation of the satellite with respect to the earth and to space. Diverse techniques are implemented to quantify these orbital parameters and carry out the operations necessary to maintain the satellite in a sought-after zone and according to a desired orientation. Geostationary satellites stabilized in relation to three axes, oriented towards the earth to allow the operation of diverse instruments such as telecommunications systems, are for example known. Attitude and orbit control systems, generally dubbed by their acronym AOCS, require that the position and the orientation of the satellite be known with high precision. Accordingly, AOCS systems implement several technologies of trackers such as a star tracker, a sun tracker or else an earth tracker.
In a known manner, the star tracker comprises means of optical measurement (for example, a CCD sensor) making it possible to take images of the celestial canopy, and a unit for processing these images making it possible to position and orient a functional trihedron of the star tracker with respect to space. By analysing the star field imaged with the aid of an onboard star catalogue, the star tracker positions the axes of its functional trihedron in space. Knowing the position of the star tracker on the satellite, the AOCS system deduces the position and the orientation of the satellite in space.
The precision of the positioning of the satellite in space is essential to the operation of the satellite and to the progress of its mission. Any inaccuracy of alignment entails the addition of a constant bias in the pointing of the satellite. It is therefore essential to precisely position the functional trihedron of the star tracker in a reference trihedron tied to the satellite. FIG. 1 illustrates the principle of the operation of aligning the star tracker on the satellite according to the known prior art. A star tracker 10 is fixed on the structure 11 of a satellite. The star tracker comprises means of optical measurement and a processing unit making it possible to position the reference trihedron T1 of the star tracker with respect to a benchmark tied to the stars. The positioning operation consists in positioning the reference trihedron T1 of the star tracker 10 with respect to a trihedron T3 tied to the structure 11 of the satellite. Knowing the transfer function between the trihedrons T1 and T3, the AOCS system will be capable of determining the orientation of the satellite in space by means of the measurement of the orientation of the star tracker in space.
According to the known prior art, the positioning is performed in two steps. In a first step, the manufacturer of the star tracker positions the reference trihedron T1 with respect to a trihedron T2 tied to the star tracker, defined by a set of geometric markers 12. In a second step, carried out by the constructor of the satellite during a phase of assembly and testing of the satellite, generally dubbed by its acronym AIT for Assembly Integration and Testing, the star tracker is fixed on the structure of the satellite. The constructor of the satellite thereafter positions the trihedron T2 tied to the star tracker in the trihedron T3 by means of the geometric markers 12 measured by an optical measurement instrument 13 tied to the structure 11 of the satellite. The combination of the two measurements makes it possible to deduce the relation between the functional trihedron of the star tracker and the reference trihedron of the satellite serving as reference in the pointing of the satellite. This approach however suffers from limits that the present invention seeks to alleviate. In particular, this approach makes it necessary to combine two measurements carried out by two distinct industrial parties and at two distinct places, a source of inaccuracy and industrial cost overhead. This approach furthermore requires the manufacturer to implement on each star tracker a set of optical markers that are easily measurable subsequently by the constructor in the AIT phase. In addition to the cost related to the mounting of this set of optical markers, this requires the constructor to identify and communicate to the manufacturer of the star tracker the positions of the optical markers liable to be measured under AIT, without risk of interference or masking by other components of the satellite. The measurement of alignment under AIT by means of optical markers mounted directly on the apparatus also constitutes a source of inaccuracy, in the case where different tests are carried out on the tracker and the satellite between the two measurements, the measurement made by the manufacturer of the tracker and the measurement made under AIT by the constructor of the satellite. It therefore remains desirable to have a means for simply and precisely positioning the functional marker of a star tracker in a marker tied to the satellite structure on which the star tracker is mounted.